Desaturated Animation After Rendering
You are on a Mac right?

I’m guessing it is post 2015 MacBook Pro?

Blender doesn’t support the proper display devices for MacBook Pros. You’ll need my GitHub version.

TL;DR: What you are seeing in Blender is wrong, and Preview is interpreting it as correct. I’d also encourage you to not render with the sRGB OETF as it is subject to colour skew with saturated colours.

PS: If this confuses you, I’ve recently covered this very subject over at the Hitchhiker’s Guide to Digital Colour. I’d recommend starting at the first question if this guess of an answer proved correct. https://hg2dc.com/. Feel free to ask any questions.